A type of D/A converter that includes a pulse-width-modulated-signal outputting circuit for outputting a pulse-width-modulated (PWM) signal having a pulse width in accordance with a digital value of a digital signal has been known. It is also known that, in this type of D/A converter, a harmonic distortion of a playback signal component (a playback signal, here and hereafter, refers to an analog signal that is to be obtained by D/A conversion) occurs (Reference Document 1; Karsten Nielsen: “A Review and Comparison of Pulse Width Modulation (PWM) Method For Analog and Digital Input Switching Power Amplifiers”, 102nd AES convention 1997 Mar. 22-25 Preprint).
As an example of a method of removing the harmonic distortion mentioned above, a D/A converter disclosed in Reference Document 2 (Japanese Unexamined Patent Application Publication No. 2-200012 (Japanese Patent Publication No. 8-28667)) is known.
The D/A converter disclosed in Reference Document 2 has an arrangement shown in FIG. 10. That is, an input digital audio signal Di having, for example, 24 bits, input via an input terminal 1, is directly supplied to a subtractor circuit 2 and also to a distortion-correcting-component generating circuit 3.
The distortion-correcting-component generating circuit 3 generates a distortion-correcting component Dn corresponding to a harmonic distortion of a playback signal component, generated in a PWM circuit 5, which will be described later. The distortion-correcting-component generating circuit 3 supplies the distortion-correcting component Dn generated to the subtractor circuit 2, where the distortion-correcting component Dn is subtracted from the input digital signal Di. Alternatively, an adder circuit may be provided instead of the subtractor circuit 2, inverting the distortion-correcting component Dn from the distortion-correcting-component generating circuit 3 before supplying it to the adder circuit.
The digital audio signal (24 bits) from the subtractor circuit 2 is supplied to a Δ-Σ modulating circuit 4, where the number of bits is reduced while localizing quantization noise to a high-frequency side. For example, the Δ-Σ modulating circuit 4 performs third-order noise shaping on the 24-bit digital audio signal from the subtractor circuit 2, outputting a digital signal having 3 to 8 bits.
The digital signal output from the Δ-Σ modulating circuit 4 is supplied to the PWM circuit 5 constituting a D/A converter section. The PWM circuit 5 outputs a PWM signal having a pulse width in accordance with a digital value of the digital signal received having 3 to 8 bits.
The PWM circuit 5 outputs a PWM signal having a pulse width in accordance with the number of bits of the signal input thereto. For example, if the digital signal received has three bits, the PWM circuit 5 outputs a PWM signal having one of seven pulse widths in accordance with a digital value of the digital signal. FIG. 3 shows, as an example, a case of double-side modulated PWM signals. As described earlier, in the PWM circuit 5, a harmonic distortion of a playback signal component occurs. In particular, a second harmonic distortion is the largest.
The distortion-correcting-component generating circuit 3 generates a correcting component Dn corresponding to the harmonic distortion generated in the PWM circuit 5, and subtracts the correcting component Dn from the input digital signal Di in advance. Then, the harmonic distortion generated in the PWM circuit 5 is cancelled by the correcting component Dn subtracted in advance. Thus, the PWM circuit 5 outputs a PWM signal in which the harmonic distortion has been removed.
The PWM signal output from the PWM circuit 5 is supplied to a speaker via a power amplifier (output amplifier circuit) implemented by, for example, a class-D amplifier, whereby a sound is played back. The class-D amplifier includes switching elements, and switching of the switching elements is driven by the PWM signal.
According to an equation disclosed in Reference Document 1 mentioned earlier, in an output of a double-side modulated PWM signal, phase characteristics of the harmonic distortion of the playback signal component does not depend on the frequency of input signal.
However, as will be described later, the inventors have verified that the phase characteristics of the harmonic distortion component depend on normalized frequency defined as (normalized frequency)=(frequency of playback signal)/(sampling frequency), and that, in the arrangement of the D/A converter according to the related art shown in FIG. 10, although a certain effect of correction is achieved when normalized frequency is low, conversely, the distortion component could be intensified when normalized frequency is high.
In particular, when a PWM signal obtained by D/A conversion of a digital audio signal is supplied to a speaker via a class-D amplifier for acoustic playback as described above, generally, in the class-D amplifier, switching devices are operated at a high voltage and a large output current is required, inhibiting a high switching frequency (sampling frequency).
Thus, when the class-D amplifier is driven by a PWM signal, the sampling frequency must be low, so that normalized frequency of a signal in the audible band (playback signal) becomes high. Accordingly, the correcting method by the circuitry according to the related art shown in FIG. 10 does not correct a harmonic distortion effectively.
The present invention has been made in view of the above, and an object thereof is to provide a D/A converter and a digital signal processing apparatus that favorably remove a distortion even when normalized frequency is high.